Suggest using INCBIN instead of INCLUDE if a file is not valid UTF-8

[ISSOtm]

help: This looks like a binary file. Have you tried `INCBIN bg_tiles.2bpp` instead?

[Rangi42]

I've more frequently run into the opposite problem, INCBINning a file I meant to INCLUDE. Maybe it would be appropriate to do filename detection, e.g. warning for INCBIN "*.inc" or INCBIN "*.asm"; or even tracking which extensions have been INCLUDEd and INCBINned before, and warning if an extension is suddenly used in the opposite way.